A.) Field of Use
The present invention relates to catalysts, systems, and methods that are useful for treating an exhaust gas which occurs as a result of combustion of hydrocarbon fuel, such as an exhaust gas produced by diesel engines.
B.) Description of Related Art
The largest portions of most combustion exhaust gases contain relatively benign nitrogen (N2), water vapor (H2O), and carbon dioxide (CO2); but the exhaust gas also contains in relatively small part noxious and/or toxic substances, such as carbon monoxide (CO) from incomplete combustion, hydrocarbons (HC) from un-burnt fuel, nitrogen oxides (NOx) from excessive combustion temperatures, and particulate matter (mostly soot). To mitigate the environmental impact of exhaust gas released into the atmosphere, it is desirable to eliminate or reduce the amount of these undesirable components, preferably by a process that, in turn, does not generate other noxious or toxic substances.
One of the most burdensome components to remove from a vehicular exhaust gas is NOx, which includes nitric oxide (NO), nitrogen dioxide (NO2), and/or nitrous oxide (N2O). The reduction of NOx to N2 in a lean burn exhaust gas, such as that created by diesel engines, is particularly problematic because the exhaust gas contains enough oxygen to favor oxidative reactions instead of reduction. NOx can be reduced in a diesel exhaust gas, however, by a heterogenic catalysis process commonly known as Selective Catalytic Reduction (SCR). An SCR process involves the conversion of NOx, in the presence of a catalyst and with the aid of a reducing agent, into elemental nitrogen (N2) and water. In an SCR process, a gaseous reductant such as ammonia is added to an exhaust gas stream prior to contacting the exhaust gas with the SCR catalyst. The reductant is absorbed onto the catalyst and the NOx reduction reaction takes place as the gases pass through or over the catalyzed substrate. The chemical equation for stoichiometric SCR reactions using ammonia is:2NO+4NH3+2O2→3N2+6H2O2NO2+4NH3+O2→3N2+6H2ONO+NO2+2NH3→2N2+3H2O
Platinum Group Metal (PGM)-based reduction catalysts have been reported since the mid-1970's (Bosch, Catalysis Today, 1988, pg. 369) to exhibit excellent NOx reduction activity at low temperatures. These catalysts, however, have very poor selectivity for N2, typically less than 50%. (Buenos Lopez, et al., Applied Catalysis B, 2005, pg. 1). At low temperatures, e.g. about 150° C. to about 250° C., the low selectivity for N2 correlates with the formation of significant amounts of N2O; while at high temperatures, e.g. greater than about 350° C., the low selectivity correlates to the oxidation of NH3 (the desired reductant) to NOx.
Another common problem with NOx reduction systems utilizing an NH3 reductant is the release of unreacted ammonia, also referred to as “ammonia slip”. Slip can occur when catalyst temperatures are not in the optimal range for the reaction or when too much ammonia is injected into the process. An additional oxidation catalyst is typically fitted downstream of an SCR system to reduce such slip. This catalyst typically contains a PGM component either in single catalyst configuration where the catalyst acts solely as an oxidation catalyst or in a dual catalyst configuration where zoning or layering of the catalyst allows for both oxidative and reductive functionality.
PGM purportedly has been incorporated into MCM-41, a mesoporous zeolite having a pore size between 20-30 Angstroms by incipient wetness for hydrocarbon SCR. Due to the large pore size, no shape selectivity for these catalysts has been observed. Park et al. studied this phenomenon, and concluded that if Pt was incorporated into the pores of ZSM-5, and 10-ring, medium pore size zeolite, by a typical incipient wetness method, then some selectivity, e.g., conversion of NOx and N2 yield would be expected. However, the catalysts made by this method show the same conversion of NOx and N2 yield. Therefore, typical incipient wetness methods are not capable of achieving high PGM exchange or incorporate onto the walls of the molecular sieve crystal structure or into the walls by filling void space within the crystal structure itself. (Park et al, From Zeolites to Porous MOF materials—the 40th Anniversary of International Zeolite Conference, 2007).
These drawbacks of conventional PGM-based catalysts limit their practical application. There is, therefore a continuing unmet need for PGM based molecular sieve catalysts that can provide for high NOx reduction efficiency at low temperatures, high N2 selectivity, and reduced NH3 slip.